The conversion of oxygenates to olefins (OTO) is currently the subject of intense research because it has the potential for replacing the long-standing steam cracking technology that is today the industry-standard for producing world scale quantities of ethylene and propylene. The very large volumes involved suggest that substantial economic incentives exist for alternate technologies that can deliver high throughputs of light olefins in a cost efficient manner. Whereas steam cracking relies on non-selective thermal reactions of naphtha range hydrocarbons at very high temperatures, OTO exploits catalytic and micro-architectural properties of acidic molecular sieves under milder temperature conditions to produce high yields of ethylene and propylene from methanol.
Current understanding of the OTO reactions suggests a complex sequence in which three major steps can be identified: (1) an induction period leading to the formation of an active carbon pool (alkyl-aromatics), (2) alkylation-dealkylation reactions of these active intermediates leading to products, and (3) a gradual build-up of condensed ring aromatics. OTO is therefore an inherently transient chemical transformation in which the catalyst is in a continuous state of change. The ability of the catalyst to maintain high olefin yields for prolonged periods of time relies on a delicate balance between the relative rates at which the above processes take place. The formation of coke-like molecules is of singular importance because their accumulation interferes with the desired reaction sequence in a number of ways. In particular, coke renders the carbon pool inactive, lowers the rates of diffusion of reactants and products, increases the potential for undesired secondary reactions and limits catalyst life.
Over the last two decades, many catalytic materials have been identified as being useful for carrying out the OTO reactions. Crystalline molecular sieves are the preferred catalysts today because they simultaneously address the acidity and morphological requirements for the reactions. Particularly preferred materials are eight-membered ring aluminosilicates, such as those having the chabazite (CHA) framework type, as well as silicoaluminophosphates of the CHA structure, such as SAPO-34. These molecular sieves have cages that are sufficiently large to accommodate aromatic intermediates while still allowing the diffusional transport of reactants and products into and out of the crystals through regularly interconnected window apertures. By complementing such morphological characteristics with appropriate levels of acid strength and acid density, working catalysts are produced. Extensive research in this area indicates that silicoaluminophosphates are currently more effective OTO catalysts than aluminosilicates. In particular, the control of the silica to alumina molar ratio is a key requirement for the use of aluminosilicates in OTO reactions. Nevertheless, aluminosilicate zeolites continue to be explored for use in OTO and appear to have yet undiscovered potential.
Chabazite is a naturally occurring zeolite with the approximate formula Ca6Al12Si24O72. Three synthetic forms of chabazite are described in “Zeolite Molecular Sieves”, by D. W. Breck, published in 1973 by John Wiley & Sons, the complete disclosure of which is incorporated herein by reference. The three synthetic forms reported by Breck are Zeolite “K-G”, described in J. Chem. Soc., p. 2822 (1956), Barrer et al; Zeolite D, described in British Patent No. 868,846 (1961); and Zeolite R, described in U.S. Pat. No. 3,030,181 (1962).
U.S. Pat. No. 4,544,538, incorporated herein by reference, describes the synthesis of another synthetic form of chabazite, SSZ-13, using N-alkyl-3-quinuclidinol, N,N,N-tri-alkyl-1-adamantylammonium cations and/or N,N,N-trialkyl-exoaminonorbornane as a directing agent in a conventional OH− medium. According to the '538 patent, SSZ-13 typically has a silica to alumina molar ratio of 8 to 50 but it is stated that higher molar ratios can be obtained by varying the relative ratios of the reactants in the synthesis mixture and/or by treating the zeolite with chelating agents or acids to remove aluminum from the zeolite lattice. The '538 patent also discloses that the crystallization of SSZ-13 can be accelerated and the formation of undesirable contaminants can be reduced by adding seeds of SSZ-13 to the synthesis mixture.
According to Published International Application No. WO 00/06494, published Feb. 10, 2000, a colloidal suspension of seeds of the LEV structure can be used to assist in the crystallization of a number of molecular sieve structures, including LEV, FER, MOR, ERI/OFF, MAZ, OFF, ZSM-57 and CHA. Examples of CHA materials are said to include chabazite and the phosphorous containing molecular sieves SAPO-34, ALPO-34, SAPO-37, ALPO-37 and metal containing derivatives thereof.
A silica crystalline molecular sieve having the CHA framework type has been hydrothermally synthesized using N,N,N-trimethyladamantylammonium in hydroxide form as the structure-directing agent at nearly neutral pH in the presence of fluoride. See Diaz-Cabanas, M-J, Barrett, P. A., and Camblor, M. A. “Synthesis and Structure of Pure SiO2 Chabazite: the SiO2 Polymorph with the Lowest Framework Density”, Chem. Commun. 1881 (1998).
More recently, an aluminosilicate with the CHA framework type and having a silica to alumina molar ratio in excess of 100, such as from 150 to 2000, has been synthesized in the presence of fluoride ions. See U.S. Patent Application Publication No. 2003/0176751, published Sep. 18, 2003 and incorporated herein by reference.
Existing methods for synthesizing high silica aluminosilicates and all silica molecular sieves with a CHA framework-type have tended to produce materials with a large crystal size. However, small crystal materials are often desirable for catalytic use, especially where a high catalyst surface area is important, such as the conversion of oxygenates to olefins.
U.S. Pat. No. 6,079,644, incorporated herein by reference, describes a zeolite that is identified as SSZ-62 and that has a CHA framework-type and a crystal size of 0.5 micron or less. SSZ-62 is said to have a silica to alumina molar ratio in excess of 10, such as in excess of 30, but the only synthesis example produces a material with a silica to alumina molar ratio of 22.